1. Field of the Invention
This invention relates generally to cardiac stimulation leads, and more particularly to an implantable cardiac stimulation lead which employs a lead body encasing a very thin noncoiled conductor cable and an external guide tube.
2. Description of the Related Art
Prior to the advent of implantable endocardial stimulation leads, surgeons and cardiologists possessed few options for providing permanent or semi-permanent electro-physiological therapy to patients suffering from cardiac arrhythmia. In cases where drug therapy and corrective surgery were ruled out, epicardial leads used with external, and later implantable, pulse generators represented the normal clinical approach. For many patients whose arrhythmia stemmed from disruptions in electrical signal propagation at highly localized spots deep within the heart, epicardial stimulation constituted a compromise treatment.
The introduction of endocardial leads capable of transvenous implantation created a boon for many cardiac arrhythmia patients. Many individuals who formerly faced the prospects of median sternotomy or thoracotomy and reliance on epicardial stimulation for endocardially originated malfunctions could be provided with a subcutaneously implanted cardiac stimulator combined with a transvenous lead that promised to yield better cosmetic results as well as the potential for better therapy through more accurate placement of lead electrodes.
Despite the myriad of advantages associated with endocardial leads, there has always been a tradeoff associated with their usage in many patients. On the one hand, transvenous implantable leads typically yield better cosmetic results and the potential for more accurate arrhythmia therapy for patients. On the other, like any foreign body introduced into the cardiovascular system, a transvenous cardiac lead presents an obstruction to the normal flow of blood, and possibly the normal operation of one or more of the valves of the heart. This partial occlusion of a portion of the patient's cardiovascular system may result in not only a diminished blood flow, but also may lead to the formation of microemboli.
For the majority of patients, the medical advantages associated with endocardial leads strongly outweigh the attendant obstruction to normal blood flow. However, for some patients, the calculation is less clear. Pediatric patients often present blood vessels that are simply too small to accommodate conventional implantable leads, and these young patients are often the least able to adjust successfully to a diminished blood flow and/or valve function. Similarly, those patients who present occluded vessels and/or eroded valve leaves resulting from disease, injury, or some other mechanism may not be suitable candidates for transvenous implanted leads. In these types of cases, epicardial leads may present the only viable solution for the arrhythmia patient.
The magnitude of blood flow area of a given vessel obstructed by a conventional endocardial lead is a function of the diameter of the lead body. Early designs for endocardial leads consisted of an elongated lead body that included a proximal connector for connection to a pulse generator and a distally located electrode for transmitting signals to the heart. The lead body consisted of a tubular insulating sleeve that jacketed a coiled conductor wire leading from the electrode to the connector. The conductor wire was coiled in a helical fashion to leave a centrally disposed lumen through which a stylet could be inserted to manipulate the lead. The minimum overall diameter for this design is limited by the sum of the diameter of the lumen, twice the diameter of the conductor wire, and twice the wall thickness of the sleeve. An early bipolar variant incorporated two coiled conductor wires separately disposed in respective lumens. Here, the minimum diameter is a function of the sum of the diameters of both lumens, twice the diameter of the conductor wire, and twice the thickness of the sleeve. Diameters of 8 French (approximately 3 mm) (1 French=3.times.diameter in millimeters) were not uncommon.
Later lead designs incorporated a coaxial arrangement that represented an advance in miniaturization. The coaxial lead utilizes a lead body with an inner conductor wire defining a lumen, an outer conductor wire, an intermediary insulating sleeve separating the two conductor wires, and an outer insulating sleeve. The minimum diameter of the coaxial bipolar lead body is limited by the sum of the diameters of the lumen, the first conductor coil, the intermediary insulator sleeve, the second conductor coil, and the outer sleeve. Overall diameters of about 6 French (approximately 2 mm) are common with this design.
A recent improvement upon the coaxial bipolar design incorporates nested and individually insulated conductor wires that circumscribe a concentrically located lumen. This uniaxial design can be seen in the Thinline.TM. (a trademark of Sulzer Intermedics, Inc.) leads produced by Sulzer Intermedics, Inc. The diameter of the Thinline.TM. lead body is a function of the sum of the diameter of the lumen, the diameter of each of the conductor wires, and twice the wall thickness of the outer sleeve. The introduction of the Thinline.TM. lead design further reduced the minimum diameter of the lead body to about 4.7 French (approximately 1.6 mm).
Despite advances in miniaturization, there are still several disadvantages associated with conventional lead designs. Conventional lead bodies require an internal lumen that is coextensive with the lead body to accommodate an internal stylet for manipulating the lead. The diameter of the lumen often constitutes a significant portion of the overall diameter of the lead body and therefore represents a limitation on the achievable miniaturization of the lead body. Similarly, conventional lead bodies incorporate coiled conductor wires that, by definition, contribute twice their own diameters to the overall diameter of the lead body. For these reasons the smallest available conventional leads may still be too large for successful transvenous implantation in some patients.
In addition, coaxial leads are susceptible to structural failure due to a phenomenon commonly known as "subclavian crush." Subclavian crush occurs when a lead is implanted via the subclavian vein (a common transvenous entry site) and is pressed against the patient's clavicle during movement of the shoulder joint. The pressing force may bend the coils of the lead wire to fracture. The problem is exacerbated if the patient suffers an externally applied trauma in the clavicle area.
The present invention is directed to overcoming or minimizing one or more of the foregoing disadvantages.